It is well recognized that diodes forward biased above their saturation current have an exponential voltage versus current characteristic. This characteristic has been widely utilized in the past to enable a logarithmic output to be derived in response to an input signal. The use of a diode, per se, for deriving a logarithmic output signal is not susceptible for use in connection with signals having wide amplitude variations. This is because the impedance of a forward biased diode changes greatly as the diode voltage varies widely, resulting in significant variations in the loading of a source driving the diode. Of course, such variations are undesirable because they result in variations in the voltage applied by the source to the diode and inaccuracies in the logarithmic output response.
One conventional, prior art circuit for converting an input voltage (having a value V.sub.s) to an output voltage that is proportional to the logarithm of the input voltage includes a DC operational amplifier having an inverting input terminal responsive to an input voltage, as coupled through a series resistor (having a value R.sub.1). The inverting input terminal is also responsive to a negative feedback voltage that is coupled from the output of the amplifier through a diode having its cathode and anode respectively connected to the output and complementary input terminals. For accurate operation of this circuit, it is necessary for the current (V.sub.s /R.sub.1) supplied by the input voltage to the complementary input terminal to be much greater than I.sub.o, the reverse saturation current of the feedback diode. Because of this requirement, the input voltage has a lower limit of 20 to 30 millivolts or the value of R.sub.1 must be so low that the input voltage source is loaded. Because of the relatively high lower limit of the input voltage source, this prior art circuit has not been capable of handling wide variations in input voltages.
Other conventional logarithmic circuits exist, such as linear operational amplifiers combined with discrete transistor circuitry. However, these arrangements are for the purpose of limiting temperature dependence of the logarithmic circuits and the diode loading problems inherent with the previously described circuit are present.
Another prior art approach to increasing the dynamic range of input signals which a logarithmic circuit can handle is to use a constant current source which drives a high impedance input terminal of a differential amplifier which is shunted by a reverse biased diode, the impedance of which varies logarithmically as a function of the current flowing through it. Thereby, the voltage across the diode is proportional to the logarithm of the current flowing through it. The amplifier functions as a voltage follower, whereby the voltage across the diode is reflected at the output of the amplifier as a voltage proportional to the logarithm of the current flowing through the diode.
The constant current source used by these prior art devices is a PIN diode or a photomultiplier, either of which is positioned to respond to a modulated optical source. The optical source may be a constant intensity light source that is modulated by the density of photographic film or the like. In the alternative, a voltage source can be applied to a diode which produces light energy proportional to the current flowing through it. Light emitted from the current responsive diode is optically coupled to the PIN diode which derives an output current proportional to the light impinging on it. Hence, this prior art approach has obvious disadvantages in converting an input voltage to an output signal that is proportional to the logarithm of the input voltage, because of the requirement for current to optical energy transducers and optical energy to current transducers.
It is, accordingly, an object of the present invention to provide a new and improved circuit for converting input voltages susceptible to wide variations in amplitude to an output voltage that is proportional to the logarithm of the input voltage.
Another object of the invention is to provide a new and improved circuit utilizing only electric circuit components to convert an input voltage to an output voltage that is proportional to the logarithm of the input voltage over a wide range of the input voltage.